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    <p>I need some help to complete my idea about regexes.</p> <h2>Introduction</h2> <p>There was a <a href="https://stackoverflow.com/questions/1579023/design-of-an-alternative-fluent-interface-for-regular-expressions">question about better syntax</a> for regexes on SE, but I don't think I'd use the fluent syntax. It's surely nice for newbies, but in case of a complicated regex, you replace a line of gibberish by a whole page of slightly better gibberish. I like the <a href="http://martinfowler.com/bliki/ComposedRegex.html" rel="nofollow noreferrer">approach by Martin Fowler</a>, where a regex gets composed of smaller pieces. His solution is readable, but hand-made; he proposes a smart way to build a complicated regex instead of a class supporting it.</p> <p>I'm trying to make it to a class using something like (see his example first)</p> <pre><code>final MyPattern pattern = MyPattern.builder() .caseInsensitive() .define("numberOfPoints", "\\d+") .define("numberOfNights", "\\d+") .define("hotelName", ".*") .define(' ', "\\s+") .build("score `numberOfPoints` for `numberOfNights` nights? at `hotelName`"); MyMatcher m = pattern.matcher("Score 400 FOR 2 nights at Minas Tirith Airport"); System.out.println(m.group("numberOfPoints")); // prints 400 </code></pre> <p>where fluent syntax is used for combining regexes extended as follows:</p> <ul> <li>define named patterns and use them by enclosing in backticks <ul> <li><code>`name`</code> creates a named group <ul> <li>mnemonics: shell captures the result of the command enclosed in backticks</li> </ul></li> <li><code>`:name`</code> creates a non-capturing group <ul> <li>mnemonics: similar to <code>(?:</code>...<code>)</code></li> </ul></li> <li><code>`-name`</code> creates a backreference <ul> <li>mnemonics: the dash connects it to the previous occurrence</li> </ul></li> </ul></li> <li>redefine individual characters and use it everywhere unless quoted <ul> <li>here only some characters (e.g., <code>~ @#%</code>") are allowed <ul> <li>redefining <code>+</code> or <code>(</code> would be extremely confusing, so it's not allowed</li> <li>redefining space to mean any spacing is very natural in the example above</li> <li>redefining a character could make the pattern more compact, which is good unless overused</li> <li>e.g., using something like <code>define('#', "\\\\")</code> for matching backslashes could make the pattern much readable</li> </ul></li> </ul></li> <li>redefine some quoted sequences like <code>\s</code> or <code>\w</code> <ul> <li>the standard definitions are <a href="https://stackoverflow.com/questions/4304928/unicode-equivalents-for-w-and-b-in-java-regular-expressions/4307261#4307261">not Unicode conform</a></li> <li>sometimes you might have you own idea what a word or space is</li> </ul></li> </ul> <p>The named patterns serves as a sort of local variables helping to decompose a complicated expression into small and easy to understand pieces. A proper naming pattern makes often a comment unnecessary.</p> <h2>Questions</h2> <p>The above shouldn't be hard to implement (I did already most of it) and could be really useful, I hope. <em>Do you think so?</em></p> <p>However, I'm not sure how it should behave inside of brackets, sometimes it's meaningful to use the definitions and sometimes not, e.g. in</p> <pre><code>.define(' ', "\\s") // a blank character .define('~', "/\**[^*]+\*/") // an inline comment (simplified) .define("something", "[ ~\\d]") </code></pre> <p>expanding the space to <code>\s</code> makes sense, but expanding the tilde doesn't. <em>Maybe there should be a separate syntax to define own character classes somehow?</em></p> <p><em>Can you think of some examples where the named pattern are very useful or not useful at all?</em> I'd need some border cases and some ideas for improvement.</p> <h1>Reaction to tchrist's answer</h1> <h2>Comments to his objections</h2> <ol> <li>Lack of multiline pattern strings. <ul> <li>There are no multiline strings in Java, which I'd like to change, but can not.</li> </ul></li> <li>Freedom from insanely onerous and error-prone double-backslashing... <ul> <li>This is again something I can't do, I can only offer a workaround, s. below.</li> </ul></li> <li>Lack of compile-time exceptions on invalid regex literals, and lack of compile-time caching of correctly compiled regex literals. <ul> <li>As regexes are just a part of the standard library and not of the language itself, there's nothing what can done here.</li> </ul></li> <li>No debugging or profiling facilities. <ul> <li>I can do nothing here.</li> </ul></li> <li>Lack of compliance with UTS#18. <ul> <li>This can be easily solved by redefining the corresponding patterns as I proposed. It's not perfect, since in debugger you'll see the blowed up replacements.</li> </ul></li> </ol> <p>I looks like you don't like Java. I'd be happy to see some syntax improvements there, but there's nothing I can do about it. I'm looking for something working with current Java.</p> <h2>RFC 5322</h2> <p>Your example can be easily written using my syntax:</p> <pre><code>final MyPattern pattern = MyPattern.builder() .define(" ", "") // ignore spaces .useForBackslash('#') // (1): see (2) .define("address", "`mailbox` | `group`") .define("WSP", "[\u0020\u0009]") .define("DQUOTE", "\"") .define("CRLF", "\r\n") .define("DIGIT", "[0-9]") .define("ALPHA", "[A-Za-z]") .define("NO_WS_CTL", "[\u0001-\u0008\u000b\u000c\u000e-\u001f\u007f]") // No whitespace control ... .define("domain_literal", "`CFWS`? #[ (?: `FWS`? `dcontent`)* `FWS`? #] `CFWS1?") // (2): see (1) ... .define("group", "`display_name` : (?:`mailbox_list` | `CFWS`)? ; `CFWS`?") .define("angle_addr", "`CFWS`? &lt; `addr_spec` `CFWS`?") .define("name_addr", "`display_name`? `angle_addr`") .define("mailbox", "`name_addr` | `addr_spec`") .define("address", "`mailbox` | `group`") .build("`address`"); </code></pre> <h2>Disadvantages</h2> <p>While rewriting your example I encountered the following issues:</p> <ul> <li>As there are no <code>\xdd</code> escape sequences <code>\udddd</code> must be used</li> <li>Using another character instead of backslash is a bit strange</li> <li>As I prefer to write it bottom-up, I had to take your lines reverted</li> <li>Without much idea what it does, I except myself having done some errors</li> </ul> <p>On the bright side: - Ignoring spaces is no problem - Comments are no problem - The readability is good</p> <p>And most important: <strong><em>It's plain Java and uses the existing regex-engine as is.</em></strong></p>
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    1. COJava 1.7 has named captures. `(?<name> ...)` defines one and `\k<name>` backreferences it, just as in Perl.
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    2. COI know... but they're usable usable for capturing only, not for defining subexpressions to be used later.
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    3. COmaaartinus: You can’t use them later because Java doesn’t have a `(?(DEFINE)…)` block yet. That’s really all you need to make that work; then you could call them with `(?&name)`. I have longer examples of these [here in the second, longer example](http://stackoverflow.com/questions/4284176/doubt-in-parsing-data-in-perl-where-am-i-going-wrong/4286326#4286326). They’re incredibly useful, but you should see that already in the BNF example I already gave.
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